Are you referring to one PC, small group of machines, or an enterprise?
If a single PC, as long as the defs are updated, it makes no real difference. If you prefer to see details of you AV client, McAfee and Symantec are both very detailed whereas Trend does not show a lot. Another downside of trend is their defintion scheme...numbers instead of version and date as symantec does it. It's much easer to comprehend a date than a definition version number.
For small groups and even an enterprise, I would say that you would want to have different clients on your Servers than you do on your clients. This is just a precautionary measure. Maybe trend didn't catch it on the mail server and NAV/McAfee did on the PC side. Also, there are standardization issues which most corporations are dealing with. Yeah, one AV solution is nice for troubleshooting and familiarity, but being "double-protected" is far better.
Just my $.02 worth.